Peter, they were basically the same thing as the emperor starts acting on their own despite the senate's demands or votes after Julius Caesar was declared a living God and was allowed to remain emperor until his death "dictator in perpetuity" - that marked the beginning of the fall of the senate. When Augustus Caesar claimed sole power to the empire, he started the cult of personality. After him, all the rest of the Roman emperors were of the Caesar bloodline and ruled until their deaths. How different is that from the word "King" eventhough the romans hated the word, they essentially had one, despite how they orginnal chosen their emperors at the start through election. Both Nero and Caligula were essentially kings in the point that, at that time, they could hold that position until death because the Roman senate had empire ceased holding periodic elections to elect a new emperor.